Advanced high-temperature superconducting magnet for fusion reactors: Segment fabrication and joint technique

Superconducting magnets may potentially become larger and more complex for use in future demonstration and commercial fusion reactors. For such situations, the innovative design concept of remountable (or demountable) high-temperature superconducting (HTS) magnets has been proposed for both tokamak and helical reactors that uses the HTS material features of high thermal stability and low cryogenic power. In Japan, the derived concept of joint-winding of HTS coils has been proposed, in which the coils are wound by connecting short conductor segments. This paper first briefly summarizes the design proposal history and current research and development (R&D) status of the important technologies for joint design, and then reports recent R&D progress in mechanical lap joints with inserted indium foils. Based on the joint resistance as a function of temperature, the applied magnetic field and the numbers of layers and rows of HTS tapes in the conductor, the joint resistance of a 100-kA-class HTS conductor joint can be reduced to one third of its initial value by low-temperature heat treatment. Mechanical behavior of these joints indicates that stress relaxation of indium does not affect the fabrication time and that mechanical joints containing indium are preferable to soldered joints under high electromagnetic forces.